A typical direct view LCD, which includes an illumination device, is discussed in U.S. Pat. No. 4,212,048. In the LCD discussed in this patent, a display is illuminated using an illumination device, which consists of a wedge-shaped transparent plate and a light source. The illumination device typically is equipped with a dichroic polarizing means to provide polarized light. A drawback of the polarized light illumination device discussed in U.S. Pat. No. 4,212,048 is that at least 50% of the light supplied by the light source is lost without being able to contribute to the formation of the image, because the polarizer absorbs the unwanted light component. Consequently, the power consumption of the display is high which limits the working time between battery replacements or battery charges. The brightness of the display is also adversely affected.
Several non-absorbing polarizing means for use in direct view displays have been developed which transmit one polarization direction of incident unpolarized light and reflect or scatter the other polarization direction of light. Several examples of such polarizers are disclosed, e.g., see PCT WO 97/41484. The reflected or scattered light is subsequently recycled through depolarization and re-direction, which enhances the efficiency of the process of conversion of unpolarized light into polarized light. The main disadvantages of these illumination devices are the high cost of the additional components such as the reflecting polarizing sheets, diffusers and quarter-wave foils and the complexity of the device.
An illumination device which attempts to address some of the above described disadvantages of direct view displays is disclosed in U.S. Pat. No. 5,587,816. Also here an expensive polarized light separating sheet is required which transmits one polarization direction of light and reflects the other, and several additional optical components are needed such as light out-coupling means and quarter-wave foils.
An illumination device which attempts to address some of the above described disadvantages is discussed in U.S. Pat. No. 5,845,035. This patent discloses a system that comprises two layers, i.e., an isotropic layer of refractive index ni, and an anisotropic layer characterized by two refractive indices in perpendicular directions, respectively ne and no, where, dependent on the specific configuration, ni must be equal to ne or no. This illumination device, however, has several shortcomings. In one specific embodiment, the light is coupled into the anisotropic layer and such an anisotropic waveguide is difficult to manufacture. In a second and preferred embodiment, the light is coupled into the isotropic layer. Here, the isotropic refractive index of the waveguide needs to be identical to the highest refractive index of the anisotropic layer. In practice, polymers are required for the isotropic material with a refractive index between 1.65 and 2.00 if drawn PET or PEN films are used. Polymers with such a high isotropic refractive index are not available commercially and, in fact, their manufacturing is far from trivial.
It is an object of the present invention to provide a polarizing device for the use, for example, in illumination devices whereby commercial, low-cost polymers can be used for the production thereof.
It is a further object of the present invention to provide a polarizing device capable of transforming a substantial portion of unpolarized light emitted by a light source into polarized light.
It is a further object of the present invention to provide a polarizing device which comprises an isotropic waveguiding layer in which the light is coupled in and a birefringent, polarization separating layer.
Another object is to provide a polarizing device wherein the waveguiding layer is isotropic wherein the separating layer comprises an anisotropic film, and wherein the isotropic waveguiding layer has a refractive index between the refractive indices of the anisotropic region.
Another objective is to provide a polarizing device wherein a commercial and low cost polymer is used for the isotropic waveguiding layer.
In U.S. Pat. No. 5,845,035, an anisotropic, birefringent polarization separating layer is required comprising a layer with a well defined molecular orientation which is obtained, for instance, by producing a stretched PET or PEN film. In general, the production of such films is notoriously difficult and an extensive and highly optimized infrastructure is required for their production.
It is an object of the present invention is to provide a polarizing device wherein the separating layer comprises a form birefringent region.
It is yet another object of the invention to provide a polarizing device wherein the separating layer comprises a form birefringent region and also comprising a light coupling-out means.
It is yet another object of the invention to provide a method for making a polarizing device comprising a separating layer and a waveguiding layer.
Yet another object of the present invention is to provide a polarizing device comprising a separating layer and comprising a waveguiding layer where the two different parts can both be injection molded.
It is a further object of the present invention to provide a polarizing device comprising two parts, which can be produced from inexpensive, commercial materials which both can be injection molded.
Furthermore, it is an object of the invention to provide an illumination system comprising a polarizing device according to the invention.
Yet another object of the present invention is to provide a flat panel picture display comprising an illumination system comprising a polarizing device according to the invention.